This application claims priority to an application entitled xe2x80x9cFlame Stabilizer for Burner for Flame Hydrolysis Depositionxe2x80x9d filed in the Korean Industrial Property Office on Jul. 21, 2001 and assigned Ser. No. 2001-44004, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to a fine particle fabricating burner using flame hydrolysis deposition (FHD), and in particular, to a flame stabilizer for a burner to stably form a thin film with fine particles to be used as a light waveguide on a wafer by flame.
2. Description of the Related Art
In general, FHD is a technology developed from fiber preform techniques and modified to produce planar waveguide geometry. A vapor mixture of halides (SiCl4, GeCl4, etc) is reacted in an oxy-hydrogen flame to form fine glass particles, which are deposited directly onto a suitable substrate, e.g., silicon. The high-silica technology offers the potential of integrating a number of passive functions on a silicon chip, as well as the possibility of hybrid integration of both active and passive devices onto a silicon motherboard.
Flame-using fine particle formation is performed by oxygen-hydrogen flames generated from a co-flow diffusion flame burner having a plurality of concentric nozzles. Being fed with flames, a source material forms fine particles by means of flame hydrolysis or oxidation. The fine particles migrate with the flames, grown by collision-caused coagulation, and adhere to a substrate in various forms by thermophoresis. The fine particles then are sintered and dried according to their use, for example, by OVD (Outside Vapor Deposition) or VAD (Vapor-phase Axial Deposition) in fabricating an optical fiber, and FHD in fabricating a planar optical waveguide thin film.
Planar optical waveguide technology has added the growth of the Internet and multimedia communications. These areas are also driving the demand for more and more capacity on networks, and the medium of choice for high-bandwidth is optical fiber. There are three technologies to increase the overall data rate in an optical link: space division multiplexing (SDM) using multiple fibers in parallel, time division multiplexing (TDM), i.e. higher bit rate per channel and fiber, and wavelength division multiplexing (WDM), i.e., more channels per fiber by making use of different wavelengths for the transport of data.
Long-haul backbone networks already use all three strategies to exploit the enormous bandwidth of the optical fiber. Transmission rates in the Terabit per second (Tb/s) range over more than 100 WDM channels have been demonstrated in laboratories around the world.
Planar optical waveguide technology, for example, enables functions such as adding and filtering out specific wavelengths to be performed efficiently and flexibly at low cost.
FIGS. 1 and 2 are schematic views of a co-flow diffusion flame burner 100 having a plurality of concentric nozzles for use in the conventional FHD process. The term xe2x80x98diffusion flamexe2x80x99 can imply that the fuel and oxidizer (air) gas streams mix together in a chemical reaction zone by mass diffusion.
Referring to FIGS. 1 and 2, a flame F is generated by spraying source materials and a carrier gas from the center of the co-flow diffusion flame burner 100 in a direction A, with combustion of hydrogen (H2) and oxygen (O2) outside the center, respectively in directions B and C. The source materials for forming particles usually include SiCl4, GeCl4, and POCl3. Since these materials are liquid at room temperature, they are used after bubbling by the carrier gas. Hydrolysis occurs where H2O or OH resulting from the combustion of H2 and O2 meets the source materials diffused from the center of the burner 100. The resulting particles migrate with the flame F and stick to a silicon wafer W by thermophoresis.
The particles generated and grown in the flame F are as small as several nanometers to tens of nanometers. For particles this small, the influence of inertia can be neglected. The velocity of the particles in the flame F is the sum of the velocity of the gas and a thermophoretic velocity over a temperature gradient.
Therefore, in the case where the flow of the flame F is a laminar flow that can define a streamline, the particles in the flame F show a similar velocity distribution. On the other hand, if the flame flow is a turbulent flow with velocity fluctuation that cannot define a streamline, the particles in the flame F move at an unstable velocity along with the flow. As a result, the deposition efficiency and deposition uniformity of the particles is reduced.
The co-flow diffusion flame F generated from the burner 100 is generally unstable for two reasons. First, a shear flow is formed due to the velocity difference between the gases sprayed from the concentric nozzles and determined by process conditions, that is, formation of the flame F, source materials, and the flow rate of the carrier gas. The second reason is that entrainment of an ambient gas and a shear flow caused by the velocity and pressure differences between the flame F and the ambient gas. The entrainment of the ambient gas and the shear flow depend on the flow outside the flame F.
In the general FHD process, the flame F attracted by gravitation is positioned by rotation or linear movement over the silicon wafer W having a relative velocity in a direction D with respect to the burner 100. An exhaust flow is absorbed in a direction E into a discharge tube 12 for discharging non-deposited particles. The flame F exists in the vicinity of the flame F. A natural convection phenomenon is observed due to heating of the silicon wafer W and the flame F itself is influenced by buoyancy in a direction G. Hence, the flame F and the flow around the flame F are vulnerable to instability.
A flame shield tube 10 is usually provided to the periphery of the co-flow diffusion flame burner 100 to prevent introduction of ambient air and thus stabilize the flame F.
However, this method is not effective in fundamentally overcoming the instability of the flame F formed after passing through the flame shield tube 10. Therefore, high particle deposition efficiency and uniformity cannot be expected. Moreover, a vortex arising from an unstable flow causes contamination by allowing glass soot particles that are not deposited on the silicon wafer W to stick to the outside of the burner 100 or inside a deposition chamber. For at least these reasons, an improved burner design is needed
It is, therefore, an object of the present invention to provide an improved flame burner.
It is another object of the present invention to provide a flame stabilizer for a burner, which stabilizes a flow around a co-flow diffusion flame to prevent the disturbance of the flame and a particle flow in the flame caused by the ambient flow with increased particle deposition efficiency and stable and uniform particle deposition.
It is another object of the present invention to provide a flame stabilizer for a burner, which prevents the burner and a deposition chamber from being contaminated with glass soot due to a vortex and a flow disturbance around a flame.
The foregoing and other objects of the present invention are achieved by providing a flame stabilizing apparatus for application to a burner for an FHD process. In the flame stabilizing apparatus, a co-flow diffusion flame burner emits a flame onto a substrate and a flame stabilizer, installed around the burner coaxially, isolates the emitted flame from an instable ambient flow to stabilize the flame and a particle flow.
Another aspect of the present invention relates to a method for stabilizing a flame from an instable ambient flow and particle flow. The method includes the steps of inserting source materials into a center of a co-flow diffusion flame burner and inserting an inert gas that is not involved in flame hydrolysis through a plurality of channels juxtaposed to the center of the burner. The method also includes the step of allowing the insert gas to develop into a flow in the same direction as an emitted flame.